1. Field of the Invention
The present invention relates to a magnetic recording head used for thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a head gimbal assembly (HGA) provided with the head, and to a magnetic recording apparatus provided with the HGA. Furthermore, the present invention relates to a control of light-emitting operation of a light source that emits light for thermal-assist, and to a method for screening thermally-assisted magnetic recording heads.
2. Description of the Related Art
As the recording densities of magnetic recording apparatuses become higher, as represented by magnetic disk apparatuses, further improvement has been required in the performance of thin-film magnetic heads and magnetic recording media. As the thin-film magnetic heads, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording media are generally a kind of discontinuous body of magnetic grains gathered together, and each of the magnetic grains has a single magnetic domain structure. Here, one record bit consists of a plurality of the magnetic grains. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic grains and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic grains raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic grains. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, the intensity of write field generated from the thin-film magnetic head is limited almost by the amount of saturation magnetic flux density of the soft-magnetic material of which the magnetic core of the head is formed. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large energy KU is used so as to stabilize the magnetization, then anisotropic magnetic field of a portion of the medium, where data is to be written, is reduced by heating the portion; just after that, writing is performed by applying write field to the heated portion.
In this thermally-assisted magnetic recording technique, there has been generally used a method in which a magnetic recording medium is irradiated and thus heated with a light such as near-field light. In this case, it is significantly important to stably supply a light with a sufficiently high intensity at a desired position on the magnetic recording medium. However, from the beginning, more significant problem to be solved exists in where and how a light source with a sufficiently high output of light should be disposed inside a head.
As for the setting of the light source, for example, U.S. Pat. No. 7,538,978 B2 discloses a configuration in which a laser unit including a laser diode is mounted on the back surface of a slider, and UP Patent Publication No. 2008/0056073 A1 discloses a configuration in which a structure of a laser diode element with a monolithically integrated reflection mirror is mounted on the back surface of a slider. Further, UP Patent Publication No. 2005/0213436 A1 discloses a structure of slider that is formed together with a semiconductor laser, and Robert E. Rottmayer et al. “Heat-Assisted Magnetic Recording” IEEE TRANSACTIONS ON MAGNETICS, Vol. 42, No. 10, p. 2417-2421 (2006) discloses a configuration in which a diffraction grating is irradiated with a light generated from a laser unit provided within a drive apparatus. Further, UP Patent Publication No. 2008/0002298 A1 and U.S. Pat. No. 5,946,281 A disclose heads in which a surface-emitting laser diode as a light source is disposed in the element-integration surface of a slider substrate.
While various types of installations of light sources as described above have been proposed, all of those laser diodes are semiconductor devices. Outputs of semiconductor devices vary with ambient temperature. In particular, ambient temperature at which magnetic disk units are used is assumed to be in the range from −5 to 60° C., for example. Accordingly, the range of variation in light output of a light source contained in the head needs to be estimated to be considerably large. Furthermore, since the range of variation in temperature of the light source due to heat generated by surrounding elements and the light source itself is also considerably large, the range of variation in light output of the light source further increases. Therefore, in order to allow the light source to stably output a light with a constant intensity at varying ambient temperatures, variations in light output of the light source needs to be detected in real time to maintain a constant light output of the light source. However, little has been done to improve monitoring of the light output from the light source for thermal-assist.
When an edge-emitting laser diode is used as the light source, laser light emitted from the end surface on the opposite side to the end surface from which a light for thermal-assist is emitted can be used as a monitoring light to monitor the light output of the light source. For example, U.S. Pat. No. 4,860,276 discloses a technique in which a monitoring light emitted from an edge-emitting laser diode in an optical recording head is received by an edge-receiving photodiode to monitor the light output. However, it is extremely difficult to provide the combination of the light source and the photodiode on the element-integration surface of a head for thermal-assist together with the optical system of the head, because of the limited area of the element-integration surface. In fact, the area of the element-integration surface of a Femto slider commonly used as a head slider today is as small as 230 μm (micrometers)×700 μm. It may be contemplated to integrate the light source and the photodiode into one chip and then to mount the chip on the element-integration surface. However, this is not preferable because fabricating into one chip adds a great deal of cost. It is also very difficult to ensure a sufficient intensity of monitoring light emitted from the opposite edge of an edge-emitting laser diode without significantly reducing the intensity of output light for thermal-assist emitted from the light source.